In general, an optical fiber sensor is a sensor which estimates a measured amount by using intensity of light passing through an optical fiber, a refractive index and a length of the optical fiber, a mode and a variation in polarization.
A main component of an optical fiber is quartz glass. An optical fiber sensor includes a core positioned at a central portion of the optical fiber and added with germanium to slightly increase a refractive index, and cladding which is layers covering the core to protect the core.
The light incident upon the optical fiber core is reflected on the interface between a high-refractive core layer and a low-refractive cladding layer to propagate along the optical fiber core.
The optical fiber sensor is classified as an intention type, a phase type, a diffraction grating type, a mode modulation type, a polarization type, a distribution measurement type, etc., and provides various measurement values such as a voltage, a current, temperature, pressure, strain, a rotation rate, sound, a gas concentration, etc.
The optical fiber sensor can perform a super-precise wideband measurement and is not influenced by electromagnetic waves. In addition, the optical sensor easily performs a remote measurement, and a sensor part thereof does not use electricity. The optical fiber is made of a silica material having excellent decay resistance so that the optical fiber is not limited by a use environment.
There is an optical sensor of a fiber Bragg grating sensor type as a typical optical fiber sensor.
The fiber Bragg grating sensor is a sensor that includes several Bragg gratings formed on one strand of an optical fiber while be spaced part from each other by a predetermined interval and uses a property that the wavelength of light reflected on each grating is changed according to a variation of external condition such as temperature or strength.
Thus, when the fiber Bragg grating sensor is deformed when physical force is operated on the optical fiber having the gratings, so that the light refraction on the gratings are changed. The strain of a structure to which the optical fiber is fixed is measured by measuring the strain of the optical fiber through the refraction measurement, so that the load and stress on the structure may be sensed.
The fiber Bragg grating sensor, of which the refractive index of the core of the optical fiber is changed to have a predetermined period, selectively reflects only a specific wavelength of light.
The fiber Bragg grating sensor has a typical wavelength value and is not influenced by electromagnetic waves. In addition, the fiber Bragg grating sensor, which is an excellent physical quantity measurement device, has been substituted for a conventional electric gage, and the application scope thereof has been widened.
Thus, the fiber Bragg grating sensor has been used as a sensor for sensing strain, an angle, acceleration, a displacement, temperature, a variation in pressure, etc., by using a total-reflection principle that the light in an angle range is fully reflected on the interface when light propagates from high-refractive material to a low-refractive material.
For example, several techniques using an FBG sensor have been disclosed in Patent Document 1 (Korean Patent Registered No. 10-1057309 issued on Aug. 16, 2011) and Patent Document 2 (Korean Patent Registered No. 10-00992628 issued on Nov. 5, 2010) by the same applicant as the present application.